but maths is maths however you learn it
There is so much to maths that they don't really cover in the curriculum.
So, yes they learn about prime numbers. But they don't learn about hunting for primes, or usefulness, or testing for them.
I'm not even sure they learn how to test for divisibility by 11 these days.
Number bases aren't on the curriculum anywhere (something I did at primary level 40 years ago). So converting between decimal and base 2 and back again, or using hexadecimal (both used in computing).
You could blow his mind with infinity, and how some infinities are bigger than others. Or, (far far too young really), if he knows about square roots then the square root of -1 is 'i'.
At y1 he'll probably be too young for these, but if he has an interest in maths I'd foster it without trying to just 'get ahead'.
However I'd endorse everyone saying time tables. Up to 12, forwards, backwards and inside out. If he's very keen then also squares to 20 as well. I have this week been doing times tables with my 14 yo (who just cannot keep them in her head). Maths is just so much harder without them at your fingertips.
Dealing confidently with negative numbers is also really useful.
As is estimating, and real life quantities.
Di you know that 1ml is the same as 1cm^3 and 1ml of water weighs 1g?